duffle export should not require invocation image to be pushed

[itowlson]

If the invocation image has not been pushed, duffle export fails with:

Error: Error preparing artifacts: Error response from daemon: pull access denied for itowlson/itowlsonhelloplusplus-cnab, repository does not exist or may require 'docker login'

From discussions with @michelleN it sounds like this is not intended behaviour.

[michelleN]

Yeah if the image is available locally, we should use that instead of trying to pull the image thereby removing a dependency on a remote registry.

[glyn]

I am cooking up a solution in #828.